How To Use • License • Author

MTA-STS is a security standard to secure e-mail delivery. E-mail servers that send inbound e-mail to your domain will be able to detect that your e-mail server supports SMTP-over-TLS via STARTTLS (also known as Opportunistic TLS) before opening the actual connection.

In case the sending e-mail server is not able to initiate a secure connection, it will end the connection to enforce transport layer encryption. This mitigates Man-in-the-middle DNS and SMTP downgrade attacks that would allow an attacker to read or manipulate e-mail in transit.

1. Make sure you are signed in to GitHub. Then click on Use this template to create a copy to your own GitHub profile (see GitHub Docs). Don't clone the repository. You may name your repository whatever you like. For simplicity, you can name it mta-sts.<your_domain.tld>.

2. Change the file .well-known/mta-sts.txt according to your needs.

3. Create a CNAME record for mta-sts.<your_domain.tld> in your domain's DNS that points to <your_username>.github.io or <your_organization>.github.io and enable GitHub Pages.

4. Open a browser to https://mta-sts.<your_domain.tld> and make sure it does not show any certificate warnings.

5. Create a TXT record for _mta-sts.<your_domain.tld> in your domain's DNS to enable the MTA-STS policy for your domain.

   You may copy & paste this to your DNS provider:

   #HOST       #TTL    #TYPE    #VALUE
   _mta-sts    3600    TXT      "v=STSv1; id=20220317000000Z"
   

   Note that you will need to change the id= here whenever you make changes to your mta-sts.txt policy file.

6. Validate your setup, for example by using the MTA-STS Lookup by MXToolBox, or looking into your Hardenize Public Report.

Optional (but highly recommended):

7. Create another TXT record for _smtp._tls.<your_domain.tld> in your domain's DNS to enable reporting (see RFC 8460). You may copy & paste this to your DNS provider:

   #HOST         #TTL    #TYPE    #VALUE
   _smtp._tls    3600    TXT      "v=TLSRPTv1; rua=mailto:tls-rua@mailcheck.<your_domain.tld>"
   

   Note that the e-mail recipient mailbox shall be on a different domain without MTA-STS being configured. This could be a subdomain like mailcheck.<your_domain.tld>. It is also quite painful to manually deal with the reports other e-mail providers will send to you. For that particular reason, you may want to consider sending these e-mails to a 3rd-party tool like Report URI, URIports, or from other commercial providers.

   You probably want this to be the same tool you might use for DMARC reports, like DMARC Analyzer or Dmarcian.

MIT License

julian.pawlowski.me  ·  GitHub @jpawlowski  ·  Mastodon @[email protected]